The invention relates to an installation and to a process for calculating the remanent magnetization of a geological formation.
Remanent magnetization is a characteristic of geological formations and leads to a magnetic field, whose direction is dependent on the geographical position of the magnetic field at the time when said geological formation was formed. As this position has varied considerably in time, the detection of the direction of the remanent magnetization makes it possible to date geological formations. This process is of great interest for underground geological formations reached by drilling and boring operations and for which it is not easy to use other methods, which explains why remanent magnetization measurements are highly appreciated in connection with the prospecting for oil, which they make easier by supplying the age of the formations traversed.
Magnetic and electromagnetic sensors are used for measuring the magnetic field associated with the remanent magnetization. However, three magnetic fields are involved in the measurements, namely, apart from the remanent field, the field associated with the magnetic susceptibility of the rock, which is induced in the rock as a function of the local magnetic susceptibility and the geomagnetic field, due to remote magnetic influences and which preponderates.
Therefore the measurement probes are formed from several sensors, whose arrangement makes it possible to distinguish the effects of these three magnetic fields. Several systems, associated with different processes, have been proposed. In that which is closest to the present invention (illustrated in French patent 89 13199), use is made of a scalar magnetometer which measures the sum of the geomagnetic field and projections along the axis of said field of susceptibility and remanent magnetic fields. The geomagnetic field is measured separately by another scalar magnetometer, which can be positioned on the surface or at another point not far from the probe (because the time variations of the geomagnetic field scarcely vary with the distance) and a susceptibility sensor is used for separately measuring the magnetic susceptibility field.
The subtractions of the geomagnetic field and the projection of the magnetic susceptibility field from the result formed by the first scalar magnetometer makes it possible to obtain the projection of the remanent field along the axis of the geomagnetic field. Tilts of the position of the magnetic pole between the geographical north and south appear clearly when a diagraphic plot is made of the remanent magnetization along the bore-hole, because they correspond to cancelling out and changes of direction of the calculated projection of the remanent magnetization. As the dates of these tilts are known, it is possible to deduce the age of the formations passed through. However, serious practical difficulties can be encountered, because the cancelling out of projections can also result from a local reduction of the remanent magnetization, which is purely fortuitous being linked with the nature of the rock and it becomes all the more difficult to distinguish between these two causes as the measurement of the residual magnetization is subject to important uncertainties, because the intensity of the geomagnetic field preponderates (50,000 nanoteslas against about 10 or fewer nanoteslas).
It is clear that an analysis of the measurements where the number of tilts of the magnetic field is poorly evaluated would have little or no value for the dating of formations.
However, it is found that the measured remanent field is almost indistinguishable or illegible over 20% of the depth of the well.
The aim of the invention is to complete the prior art device by detecting by means of measurements and calculations another component of the remanent field, which can e.g. be perpendicular to the axis of the geomagnetic field. It is then possible to immediately establish if the cancelling out of the projection along the axis of the magnetic field is due to a tilt of the magnetic pole or to a reduction of the remanent magnetization.